Radio frequency identification devices (RFIDs) are known in the art. Such devices are typically used for inventory tracking. As large numbers of objects are moved in inventory, product manufacturing, and merchandising operations, there is a continuous challenge to accurately monitor the location and flow of objects. Additionally, there is a continuing goal to determine the location of objects in an inexpensive and streamlined manner. One way to track objects is by affixing RFID tags to objects or groups of objects, and interrogating the RFID tags with an interrogator or reader to determine which objects are present in any particular location. RFID tags may be provided with unique identification numbers or codes in order to allow a reader to distinguish between multiple different tags.
Some RFID tags use the electromagnetic field from an interrogator for power. Typically, these devices are passive (have no power supply), which results in a small and portable package.
Another type of RFID tag is an active RFID tag, which includes its own source of power, such as a battery.
If an interrogator or reader has prior knowledge of the identification number of a device, the reader can specify that a response is requested only from the device with that identification number. Sometimes, such information is not available. For example, there are occasions where a reader is attempting to determine which of multiple devices are within communication range. When the reader sends a message to a transponder device requesting a reply, there is a possibility that multiple transponder devices will attempt to respond simultaneously, causing a collision, and thus an erroneous message to be received by the reader. For example, if the interrogator sends out a command requesting that all devices within a communications range identify themselves, and receives a large number of simultaneous replies, the interrogator may not able to interpret any of these replies. Tag responses can interfere with each other and the reader sees collisions or incomprehensible noise. Therefore, arbitration or singulation schemes are employed to permit communications that are free of collisions. The term singulation refers to identifying a specific individual tag in a multiple tag environment.
In some arbitration or singulation schemes, described in commonly assigned U.S. Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all to Snodgrass et al. and the disclosures of all of which are incorporated herein by reference, a reader sends a command causing each device of a potentially large number of responding devices to select a random number from a known range and use it as that device's arbitration number. By transmitting requests for identification to various subsets of the full range of arbitration numbers, and checking for an error-free response, the interrogator quickly determines the arbitration number of every responder station capable of communicating at the same time. Thereafter, the interrogator is able to conduct subsequent uninterrupted communication with devices, one at a time, by addressing only one device. Various arbitration or singulation schemes are discussed in commonly assigned U.S. Pat. Nos. 6,275,476 to Wood, Jr.; 6,118,789 to Wood, Jr.; 6,072,801 to Wood, Jr. et al.; and 6,061,344 to Wood, Jr., the disclosures of all of which are incorporated herein by reference.
It is possible to have multiple readers operating in the same location. Problems can arise when multiple readers try to conduct an inventory at the same time.
EPCglobal is a standard setting organization that is developing standards for electronic product codes to support the use of RFID technology. One of their standards, called Class 1, Generation 2 (also known as “Gen 2”) applies to passive RFID systems. These standards evolve over time, and for a particular standard, such as Gen 2, there are minor variations between versions. The present version of the Class 1, Generation 2 standard is version 1.0.9.
The various protocols are designed to arbitrate the collisions, and EPCglobal's Gen 2 provides a specific methodology for arbitrating the collisions. In some of the above described patents, a binary-tree approach is used, in which some unique identifier is assigned to each tag, Such as the electronic product code, and the reader goes down the tree of possible numbers, until it is confident that it is talking to just a single tag. At that point the tag is read, and put to sleep. The reader then goes back up the tree and tries to singulate another tag. This repeats until there are no more tags left responding.
The Gen 2 standard takes a different approach to generating an inventory. The approach used by the Gen 2 standard, has inventory rounds of 2Q slots. Although the word “slot” is used in the specification, it is not really a timeslot. It is more of a counter. The process is illustrated in FIG. 1. At the beginning of each round, each tag sets its respective slot counter 10, 12, 14, 16, 18 to a random number, from 0 to 2Q−1. The reader sends a command that starts an inventory process. Any tag whose slot counter is 0 sends a reply; all other tags decrease their slot counter by 1. This process is repeated for all 2Q slots. Q is the number of bits, or width of the slot counter. By changing Q, the reader can optimally adjust the number of slots per round to adapt to the number of tags expected to be present. For example, if it is known that there are no more than six tags in a field, there is no need to use a slot counter that is 16 bits wide when 3 bits are sufficient. Thus Q, of the Gen 2 standard, is effectively a mask on the slot counter. Q is similar to the width of the Arbitration Mask described in the above-incorporated Snodgrass and Wood, Jr. patents.
Thus, Q is a parameter that is used to regulate the probability of tag response. During an inventory round, the tags respond only when their slot counter equals zero. The slot counter contents are derived in a pseudorandom manner based on the value of the parameter Q. In the Gen 2 standard, Q is, for example, an integer between 0 and 15, and the number of slots is between 20 and 215.
The Gen 2 standard further allows the user to include in an inventory round only tags that meet certain selection criteria. Appropriate combinations of Select commands can be used to implement Boolean criteria within a tag population.
The Gen 2 standard has also implemented a method called “Sessions” to attempt to solve the problem of two, three or four readers reading the same population of tags in the same time period; i.e., an inventory processes overlap. According to the specification, a reader shall support and tags shall provide four sessions, and tags shall participate in one and only one session during an inventory round. Two or more interrogators can use sessions to independently inventory a common tag population. Tags associate a separate and independent “inventoried” flag to each of the multiple readers. After singulating a tag, an interrogator may issue a command that causes the tag to invert its inventoried flag for that session. However, all four sessions use the same random number generator and same slot counter. When a tag's arbitration procedure gets interrupted (when one sessions stops and another session starts), the tag has to start over in an arbitration with a new reader and has to generate a new slot counter and handle (RN16) value. When the slot counter gets to zero, the tag replies and may have been inventoried by the interrupting reader. But when the first reader tries to resume its inventorying, the tag has lost its original handle and the reader cannot find the tag. The first reader has to begin its inventory process all over again.
Relevant portions of the Gen 2 specification will now be substantially repeated, to better enable one of ordinary skill in the art to understand the Gen 2 arbitration procedure. According to the Gen 2 specification, the inventory commands that a Gen2 reader or interrogator use include a “Query” command, a “QueryAdjust” command (described below), a “QueryRep” command (described below), an “ACK” (acknowledge) command, and a “NAK” (no acknowledgement or not acknowledged) command or transmission. The Query command initiates an inventory round and decides which tags participate an inventory round. A round is a period between successive Query commands.
Also according to the Gen 2 specification, the Query command contains the slot-count parameter Q. Upon receiving a Query command, participating tags pick a random number in the range of 0 to 2Q−1, inclusive, and load this value into their slot counter. Tags that pick a zero transition to the reply state and reply immediately. Tags that pick a nonzero value transition to an “arbitrate” state (see FIG. 3) and await a QueryAdjust (described below) or a QueryRep command. Assuming a single tag replies, the following occurs: a) the tag backscatters an RN16 (16 bit random or pseudo random number) as it enters a reply state, b) the reader acknowledges the tag with an ACK COMMAND containing the same RN16, c) the acknowledged tag transitions to an “acknowledged” state, and backscatters its PC (protocol control data), EPC (electronic product code data that identifies the object to which the tag is or will be attached and distinguishes the tag from other tags) and CRC-16 (cyclic redundancy check), and d) the interrogator issues a QueryAdjust or QueryRep command (not shown), causing the identified tag to invert an “inventoried” flag (i.e., from A to B or B to A) and to transition to a “ready” state, and potentially causing another tag to initiate a query-response dialog with the interrogator, starting in step (a), above. The tag can be considered to be singulated after it is acknowledged.
If the tag fails to receive the ACK acknowledgement in step (b) within a specified time, or receives the ACK with an erroneous RN16, it returns to the arbitrate state. If multiple tags reply in step (a) above, but the reader is able to resolve an RN16 from one of the tags, the reader can acknowledge the resolved tag. Unresolved tags receive erroneous RN16s from the reader and return to the arbitrate state without backscattering their PC, EPC, and CRC-16.
If the reader sends a valid acknowledgement (i.e., an ACK containing the correct RN16) to the tag in the acknowledged state, the tag re-backscatters its PC, EPC, and CRC-16.
At any point the reader may issue a NAK. In response to receiving the NAK, all tags in the inventory round return to the arbitrate state without changing their inventoried flag.
After issuing a Query command to initiate an inventory round, the reader typically issues one or more QueryAdjust or QueryRep commands. The QueryAdjust repeats a previous Query and may increment or decrement Q, but does not introduce new tags into the round. QueryRep repeats a previous Query without changing any parameters and without introducing new tags into the round. An inventory round can contain multiple QueryAdjust or QueryRep commands. At some point the reader will issue a new Query, thereby starting a new inventory round.
Tags in the arbitrate or reply states that receive a QueryAdjust first adjust Q (increment, decrement, or leave unchanged), then pick a random value in the range of 0 to 2Q−1, inclusive, and load this random value into their slot counter. Tags that pick zero transition to the reply state and reply immediately. Tags that pick a nonzero value transition to the arbitrate state and await a QueryAdjust or a QueryRep command.
Tags in the arbitrate state decrement their slot counter every time they receive a QueryRep command, transitioning to the reply state and backscattering an RN16 when their slot counter reaches 0000 (hexadecimal). Tags whose slot counter reached 0000, who replied, and who were not acknowledged (including tags that responded to the original Query and were not acknowledged) return to the arbitrate state with a slot value of 0000 and decrement this slot value from 0000 to 7FFF (hexadecimal) at the next QueryRep, thereby effectively preventing subsequent replies until the tag loads a new random value into its slot counter. Tags reply at least once in 2Q−1 QueryRep commands.
Although tag inventory is based on a random protocol, the Q-parameter affords network control by allowing a reader to regulate the probability of tag responses. Q is an integer in the range of (0,15) thus, the associated tag-response probabilities range from 20=1 to 2−15=0.000031.
To illustrate an inventory operation, the Gen 2 specification provides the following specific example. Assume a population of sixty-four powered tags in the ready state. A reader first issues a Select command to select a subpopulation of tags. Assume that sixteen tags match the selection criteria. Further, assume that twelve of the sixteen selected tags each have their inventoried flag set to A in session S0. The reader issues a Query specifying (SL, Q=4, S0, A). Each of the 12 tags having their inventoried flag set to A picks a random number in the range of (0,15) and loads the value into its slot counter. Tags that pick a zero respond immediately. The Query has three possible outcomes:
a) No tags reply. The reader may issue another Query, or it may issue a QueryAdjust or QueryRep.
b) One tag replies. See FIG. 2. The tag transitions to the reply state and backscatters 20 an RN16. The reader acknowledges the tag by sending 22 an ACK. If the tag receives the ACK with a correct RN16, it backscatters 24 its PC, EPC, and CRC-16 and transitions to the acknowledged state. If the tag receives the ACK with an incorrect RN16, it transitions to arbitrate. Assuming a successful ACK, the reader may either access the acknowledged tag or issue 26 a QueryAdjust or QueryRep to invert the tag's inventoried flag from A to B and send the tag to the ready state. A Query with a matching prior-round session parameter will also invert the inventoried flag from A to B.
c) Multiple tags reply. The reader observes a backscattered waveform comprising multiple RN16s. It may try to resolve the collision and issue an ACK; not resolve the collision and issue a QueryAdjust, QueryRep, or NAK; or quickly identify the collision and issue a QueryAdjust or QueryRep before the collided tags have finished backscattering. In the latter case, the collided tags, not observing a valid reply within the specified time, return to the arbitrate state and await the next Query of QueryAdjust command. The Gen 2 arbitration process is also described in connection with a tag state diagram included in the Gen 2 specification, a relevant portion of which is reproduced as FIG. 3. Arbitrate state 30, reply state 32, and acknowledged state 34 correspond to such states as discussed above. FIG. 3 also shows that the term “handle,” as used in the Gen 2 specification, corresponds to an RN16 that a reader uses as a nickname for a tag after a tag has a zero slot value and is acknowledged. Additional details, if desired, can be seen in the Gen 2 specification.
Instead of asking for a tag with a specific masked random number as described in the above incorporated Snodgrass and Wood, Jr. patents (and the reader subsequently going up or down through all numbers in the masked random number space, e.g., using the logic trees of the Wood, Jr. patents), the slot counters of tags based on the Gen 2 specification count down toward 0 in response to receiving each QueryRep command. When a tag slot counter reaches 0, the tag generates a new random number and returns it to the reader, and this is used as a temporary tag number or “handle” by the reader. The Gen 2 handle is substantially similar to the Random Number of the Snodgrass and Wood, Jr. patents. The Gen 2 process is substantially similar to the Snodgrass and Wood Jr. processes except that the Gen 2 tags make the countdown steps and issue new and large random numbers. This requires significantly more complexity in the tags, where real estate is expensive, instead of in the readers.
A conventional RFID tag 36, shown in FIG. 4, has one random number generator 38, one location 40 for storing random numbers, and one mask or Q value storage location 42. There is a problem when a second reader 44 initiates an arbitration process concurrent with the arbitration process of a first reader 46. The second reader 44, while it may have a different session number, must command tags to each generate a new random number and mask. The new random numbers and new masks may differ in value from those generated for the first reader arbitration. Since each tag 36 stores only one random number and one mask or Q value, the first random number and mask are lost to the tags and unavailable to the first reader 46 for use upon continuing (resuming) after a second reader's arbitration process. The first reader 46 must send commands for its tag population to generate new random numbers and perhaps different masks (Q values), both of which can lead to prolonged inventory times. The problem is exacerbated if an additional reader 48 is introduced. In some cases, the inventory processes may never finish. In other words, the EPCglobal system does not truly provide multiple concurrent inventorying, even for two sessions, let alone four.
More particularly, using conventional tags in multiple concurrent arbitrations will result in extra sortings of tags. This is because, when a second reader starts its sort, the second reader's command will cause the tags to generate new random numbers and Q values that are, at that point, lost to the first reader's sorting process. This causes the first reader to ask for a new random number that is masked to the first Q value. The first (interrupted) sorting addressing sequence is lost and must be restarted upon resuming. In the case of multiple interruptions and restarts, the arbitration process of any reader may in some cases be extended indefinitely and not converge to a conclusion.